The present invention relates to a driving circuit for a semiconductor display device using thin-film transistors. More specifically, the invention relates to a thin-film transistor circuit which uses a differential amplifier circuit and a current mirror circuit and which is used in a driving circuit for an active-matrix type semiconductor display. The invention also relates to a semiconductor display device using the thin-film transistor circuits.
A technology for manufacturing thin-film transistors (TFT) using a semiconductor film formed on an inexpensive glass substrate has advanced rapidly in recent years. This is because there are growing demands for active matrix liquid crystal displays and EL displays. The active matrix liquid crystal display has a TFT disposed in each of several tens to several millions of pixel areas arranged in matrix and controls electric charges coming into or going out of individual pixel electrodes by the switching function of the TFTs.
FIG. 11 shows a configuration of a conventional active matrix liquid crystal display device. A shift register and a buffer circuit are generally called a driving circuit and are in recent years formed on the same substrate as the active matrix circuit. Denoted 1101 is a source signal line side driving circuit and 1102 a gate signal line side driving circuit.
Designated 1103 is an active matrix circuit (pixel matrix circuit) which has pixels TFT 1104 arranged in matrix. Each pixel TFT 1104 has its drain electrode connected to a pixel electrode. Between these pixel electrodes and counter electrodes is sandwiched and sealed liquid crystal. The pixel TFTs 1104 are each formed with an auxiliary capacitor 1106 to hold charge.
A technology is also known which uses quartz as a substrate and manufactures thin-film transistors using a polycrystalline silicon film.
Another technology is also known which utilizes a laser anneal method to manufacture thin-film transistors using crystalline silicon film on a glass substrate.
In the configuration shown in FIG. 11, a timing signal from the shift register circuit of the source signal line side driving circuit selects an image signal supplied to an image line, and the selected image signal is fed to the corresponding source signal line. Further, a timing signal from the gate signal line side driving circuit is supplied to the corresponding gate signal line (scan line). The image signal fed to the source signal line is written into the pixel electrode of the thin-film transistor selected by the timing signal from the gate signal line.
This operation is repeated by setting an appropriate timing to successively write information into each pixel arranged in matrix.
After image information for one screen (one frame) has been written, the writing of image information for the next screen is performed. In this way, images are displayed one after another. Normally, the writing of image information for one screen is performed 30 or 60 times a second.
FIG. 12 shows one example of the source signal line side driving circuit. Reference number 1200 represents a clock input terminal, 1201 a clock line, 1202 a start pulse input terminal, 1203–1205 shift registers, 1206–1211 inverter type buffers, 1212 a video signal input terminal, 1213 a video signal line, 1214–1216 and 1220–1222 switches, 1217–1219 and 1225–1227 storage capacitors, 1223 a transfer signal input terminal, 1224 a transfer signal input line, 1228–1230 analog buffers, and 1231–1233 source signal line connection terminals.
In the case of an analog gray scale, a gray scale signal entered into the source signal line side driving circuit uses a video signal which is continuous in time. In the case of a normally white mode (a display mode that displays a white color when the liquid crystal is not impressed with a voltage), a setting is made such that the displayed color approaches black as the absolute value of the voltage of the gradation signal increases. To the shift registers 1203–1205, a start pulse is applied in synchronism from the start pulse input terminal 1202, with the video signal and are shifted by a clock pulse entered from the clock pulse line. The outputs of the shift registers 1203–1205 are fed through the inverter type buffers 1206–1211 to a sampling circuit.
The sampling circuit comprises switches 1214–1216 and storage capacitors 1217–1219.
Here, one example of a conventional circuit used as analog buffers 1228–1230 is shown in FIG. 13. Designated 1301 is a terminal connected with a storage capacitor and used as a signal input terminal (IN). Denoted 1302 is a terminal connected with a source signal line and used as a signal output terminal (OUT). Reference numeral 1303 represents a constant current source, 1304 a constant voltage source, 1305 and 1306 P-channel TFTs, and 1307 and 1308 N-channel TFTs. In the analog buffer of FIG. 13, the differential circuit comprises P-channel TFTs and the current mirror circuit comprises N-channel TFTs.
The operation of the analog buffer of FIG. 13 will be described. When the voltage of the input terminal (IN) 1301 of the differential circuit connected to the storage capacitor increases, the input current of the current mirror circuit connected to the opposite phase output of the input terminal (IN) 1301 decreases and the output current of the current mirror circuit also decreases correspondingly. On the other hand, the current of the same phase of the input terminal increases, causing the voltage of the output terminal (OUT) 1302 to rise to the same voltage level as the input terminal of the differential circuit. Therefore, the voltage of the source signal line connected to the output terminal (OUT) 1302 becomes equal to that of the input terminal.
In recent years, as the amount of information handled increases sharply, efforts have been made to increase the display capacity and enhance the resolution of the display. Examples of computer display resolutions for some standards are shown below in terms of pixel numbers.
Pixel number (horizontal×vertical): Standard                640×640: EGA        640×480: VGA        800×600: SVGA        1024×768: XGA        1280×1024: SXGA        
Recent years have seen the spread of software even in a personal computer field that performs a plurality of displays which are different in nature. This gives rise to a trend that a growing number of displays are compatible with XGA and SXGA standards that have higher resolutions than VGA and SVGA.
The active matrix liquid crystal displays are very frequently used in the field of notebook type personal computer. In recent years, they have come to be used not only in the notebook type personal computer but often as the displays of desktop personal computers.
In addition to the display of data signals in personal computers, the active matrix liquid crystal displays with high resolutions have come to be used for displaying television signals.
The buffers or analog buffers in the active matrix liquid crystal display devices used for such displays are not useful if their current capacity is small, and thus are required to have a certain magnitude of current capacity. When buffers or analog buffers with a large current capacity are made using thin-film transistors, the TFTs with a large current capacity, i.e., with a large channel width, are necessary. However, the TFTs with a large channel width have variations in the crystallinity among devices, which in turn causes variations in threshold voltage among TFTs. Hence, there are necessarily variations in the characteristic of the buffer or analog buffer made of a plurality of TFTs. This means that buffers or analog buffers have characteristic variations among individual source signal lines, and these characteristic variations will lead to variations in voltage applied to the pixel matrix circuit, which in turn will cause display unevenness on the entire display screen.
When the TFT size (channel width) is too large, in some cases, only the central part of TFT functions as a channel, with end portions not working as channels, thus accelerating the deterioration of TFTs.
Further, when the TFT size is large, the heat of the TFT itself increases, causing variations and degradations in the threshold.
In the gate signal line side driving circuit, too, a scanning signal is successively supplied to the gate signal line (scanning line) according to the timing signal from the shift register. The digital driving circuit that performs a sequential line driving must drive all pixel TFTs connected to one scanning line and thus the load capacitor connected to one scanning line is large. Therefore, the gate signal line side driving circuit is also required to pass the timing signal from the shift register through the buffer circuit or the like to eliminate  dulling.  In this case also, a buffer with a large current capacity is needed, which raises a problem described above. Particularly because the buffer of the gate signal line is required to drive all the TFTs in the pixel matrix circuit connected to one line, the characteristic variations will cause significant image unevenness. This is one of the most serious problems in the way to realizing high-precision, high-definition displays.
In recent years, attentions are being focused on a technology in which the semiconductor thin film is made polycrystalline by applying a laser beam against the semiconductor thin film formed on a substrate. With this technology, it is possible to impart a high level of energy that equals to a thermal anneal only to a desired localized area, offering the advantage that the whole substrate does not have to be subjected to high temperature.
Particularly a technology that renders a semiconductor thin film polycrystalline by using a pulse oscillation laser such as an excimer laser, is drawing attention. This method throws a laser pulse of large energy against the semiconductor thin film to instantly melt the semiconductor thin film, which then grows crystals as it solidifies.
There is another method which is being spotlighted. This method changes the shape of the laser beam into a linear shape longer in width than the substrate to be processed and scans this beam relatively to the substrate. The word  scan  here refers to irradiating the linear laser beam while shifting it so that the scanned paths overlap each other.
The above technique that applies a linear pulse laser beam while shifting it and overlapping the scanned paths, however, produces lines or stripes on the surface of the laser-irradiated semiconductor thin film. These stripes have great adverse effects on the characteristics of the thin-film transistors formed on the semiconductor thin film or on devices that will be formed on the semiconductor thin film. When analog buffers used for the driving circuits are to be formed, in particular, their characteristics need to be uniform as mentioned above, and thus the linear stripes pose a serious problem. In this case, within each of the stripes, characteristic is uniform but there are characteristic variations among different stripes.
Even in an anneal method using a linear laser beam, the uniformity of laser irradiation effect constitutes an issue. The high uniformity referred to here means that similar device characteristics are obtained wherever on the substrate the devices are formed. To enhance the uniformity is to make uniform the crystallinity of the semiconductor material. For enhanced uniformity, the following steps are taken.
To alleviate nonuniformity of the laser irradiation effect, it has been found that applying a weak pulse laser beam as a preliminary step (hereinafter referred to as a preliminary irradiation) before using a stronger pulse laser beam (hereinafter referred to as a main irradiation) improves the uniformity. This is very effective in suppressing variations and improving the characteristics of the semiconductor device circuits.
Why the preliminary irradiation is effective in maintaining the uniformity of the film is that the film of the semiconductor material containing amorphous portions described above has a property such that its laser energy absorption factor differs significantly from those of the polycrystalline and single crystal films. In other words, the mechanism of the two-step irradiation is that the first irradiation crystallizes the amorphous portions remaining on the film and the second irradiation promotes the overall crystallization. By slowly effecting the crystallization in this way, it is possible to suppress to a certain extent the stripe-like variations produced on the semiconductor material by the application of a linear laser beams. This countermeasure substantially improves the uniformity of the laser beam irradiation effect, making the stripe pattern relatively inconspicuous.
However, when a large number (several millions to several tens of millions) of thin-film transistors need to be formed on a glass substrate for an active matrix semiconductor display, such as a liquid crystal display, even the above-mentioned two-step laser irradiation method is not satisfactory in terms of the uniformity of irradiation effect.
It is therefore an object of the present invention to solve the problems described above and to provide a thin-film transistor circuit used in a driving circuit for a semiconductor display device that can produce a good image with high precision and high resolution and without image unevenness.